This invention relates to thin film magnetic heads, and more particularly relates to thin film structures in which two or more different heads, such as analog and digital heads, are integrated on a single substrate.
The continuing trend of miniaturization of electronic components includes magnetic heads for reading and/or writing information on magnetic media such as tape. For example, the recently proposed format for compressed digital audio tape, known as digital compact cassette or "dcc", employs tape of the same size as that used for the standard analog format known as compact cassette. However, instead of having four tracks, as does the compact cassette, the dcc tape has eighteen tracks, thus placing stringent demands on the magnetic heads which will write and read these tracks.
For example, in designing read and write heads for the dcc format, alignment of the read and write heads on the tracks is of major concern. In order to avoid reading of adjacent tracks due to tape alignment errors, the write head in such structures is typically wider than, for example, more than twice the width of, the read head. As further assurance against misalignment of the read and write heads, it has been proposed to combine these heads in a single, integrated thin film structure, such as that described and claimed in commonly assigned copending U.S. patent application Ser. No. 703,539, filed May 21, 1991.
This integration in turn creates new concerns. For example, a problem associated with such a combined digital head structure is a signal loss occurring during reading when the center of the read head is laterally displaced from the center of the tape track, even though the read head is still entirely within the bounds of the track. In parent application Ser. No. 717,301, the specification of which is incorporated herein by reference, this problem is attributed to the so-called "wrap around pole effect", in which the layers of the thin film structure above the relatively narrow layer or layers which define the width of the read head "wrap around" these relatively narrow layers, causing a vertical offset of the outer portions of the write gap, with respect to the central portion of the write gap situated above these relatively narrow layers. The parent application teaches combined read/write thin film digital head structures which substantially eliminate this "wrap around pole effect".
A significant advantage of using the same size tape as is used for the compact cassette is that the dcc cassettes and decks can be designed so that the decks are compatible with existing compact cassettes. Thus, consumers will be able to play their existing collections of compact cassettes as well as the new dcc cassettes on the new dcc decks. To achieve this compatibility, the dcc decks will have to incorporate analog read heads.
In order to preserve the advantages of integration already envisioned for the digital heads, various designs have been proposed in which the digital and analog heads are integrated on a single substrate. See, for example, copending U.S. patent application Ser. No. 919,507 (PHN 12816B), filed Jul. 24, 1992, a continuation of Ser. No. 795,280, filed Nov. 19, 1991, a continuation of Ser. No. 473,085, filed Jan. 30, 1990.
In one embodiment, a row of nine digital heads is arranged on one side of the substrate, for reading and/or writing nine digital tracks on one half of a digital tape, and two analog heads are arranged on the other side of the substrate, for reading one half of an analog tape, in one direction of tape travel. For reading and/or writing in the reverse direction, the head is rotated 180 degrees.
A portion of such an integrated head structure 10, viewed in a direction normal to the head face, is shown in FIG. 1. Located adjacent one another on a common substrate 12 of a non-magnetic material are digital and analog heads 14 and 16, respectively. Digital head 14 is a combined read/write head including lower flux guide 18 of the read portion, which conducts magnetic flux from the tape to an inwardly located magnetoresistive sense element (not shown). This lower flux guide typically has a thickness of about 0.4 microns, chosen for minimization of steps, which could lead to discontinuities in the overlying test/bias conductor. See co-pending U.S. patent application Ser. No. 703,539. The adjacent analog read head 16 includes lower flux guide 20, also having a thickness of about 0.4 microns, chosen for the same reason. Formed on top of these flux guides is an electrically insulating layer 22, typically of an oxide such as Al.sub.2 O.sub.3, in a thickness suitable to form the read gap 22A of the digital head, typically 0.4 microns. Because this layer 22 is too thin to form the analog read gap, a second electrically insulating layer 23 is formed on layer 22. The combined thickness of layers 22 and 23 define the analog read gap 23A. Following this, the upper flux guide 24 of the analog read head is formed. The read portion of the digital head 14 is completed with upper flux guide 28. This flux guide 28 also serves as the lower flux guide for the write portion of the head, and thus is sometimes referred to as the shared flux guide. Write gap-defining oxide layer 30 and upper flux guide 32 complete the write portion of the structure.
Since the width of the write gap is defined by the width of the flux guides 28 and 32, the write gap could have a step or offset .DELTA. between central region 30A and side regions 30B and 30C, caused by the conformance of layers 22 and 23 to flux guide 18, leading to the wrap around pole effect referred to above. Thus, in accordance with the teachings of parent application Ser. No. 717,301, this offset .DELTA. can be avoided by selecting the same thickness for the digital lower flux guide layer 18, and insulating layers 22 and 23, and by selectively etching the portion of layer 23 overlying flux guide 18, thereby providing an approximately planar surface for the formation of the subsequent layers of the digital head. However, some variations in layer thicknesses due to process variations are practically unavoidable, resulting in some offset. Offsets as small as 0.3 micron can have a significant wrap around pole effect on head output.